Measure of how closely a shape resembles a sphere

Sphericity

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Sphericity is a measure of how closely the shape of a physical object resembles that of a perfect sphere. For example, the sphericity of the balls inside a ball bearing determines the quality of the bearing, such as the load it can bear or the speed at which it can turn without failing. Sphericity is a specific example of a compactness measure of a shape.

Sphericity applies in three dimensions; its analogue in two dimensions, such as the cross sectional circles along a cylindrical object such as a shaft, is called roundness.

Definition

Defined by Wadell in 1935, the sphericity, $\Psi$ , of an object is the ratio of the surface area of a sphere with the same volume to the object's surface area:

\Psi ={\frac {\pi ^{\frac {1}{3}}(6V_{p})^{\frac {2}{3}}}{A_{p}}}

where $V_{p}$ is volume of the object and $A_{p}$ is the surface area. The sphericity of a sphere is unity by definition and, by the isoperimetric inequality, any shape which is not a sphere will have sphericity less than 1.

Ellipsoidal objects

The sphericity, $\Psi$ , of an oblate spheroid (similar to the shape of the planet Earth) is:

\Psi ={\frac {\pi ^{\frac {1}{3}}(6V_{p})^{\frac {2}{3}}}{A_{p}}}={\frac {2{\sqrt[{3}]{ab^{2}}}}{a+{\frac {b^{2}}{\sqrt {a^{2}-b^{2}}}}\ln {\left({\frac {a+{\sqrt {a^{2}-b^{2}}}}{b}}\right)}}},

where a and b are the semi-major and semi-minor axes respectively.

Derivation

Hakon Wadell defined sphericity as the surface area of a sphere of the same volume as the object divided by the actual surface area of the object.

First we need to write surface area of the sphere, $A_{s}$ in terms of the volume of the object being measured, $V_{p}$

A_{s}^{3}=\left(4\pi r^{2}\right)^{3}=4^{3}\pi ^{3}r^{6}=4\pi \left(4^{2}\pi ^{2}r^{6}\right)=4\pi \cdot 3^{2}\left({\frac {4^{2}\pi ^{2}}{3^{2}}}r^{6}\right)=36\pi \left({\frac {4\pi }{3}}r^{3}\right)^{2}=36\,\pi V_{p}^{2}

therefore

A_{s}=\left(36\,\pi V_{p}^{2}\right)^{\frac {1}{3}}=36^{\frac {1}{3}}\pi ^{\frac {1}{3}}V_{p}^{\frac {2}{3}}=6^{\frac {2}{3}}\pi ^{\frac {1}{3}}V_{p}^{\frac {2}{3}}=\pi ^{\frac {1}{3}}\left(6V_{p}\right)^{\frac {2}{3}}

hence we define $\Psi$ as:

\Psi ={\frac {A_{s}}{A_{p}}}={\frac {\pi ^{\frac {1}{3}}\left(6V_{p}\right)^{\frac {2}{3}}}{A_{p}}}

Sphericity of common objects

Name	Volume	Surface area	Sphericity
Sphere	${\frac {4\pi }{3}}\,r^{3}$	$4\pi \,r^{2}$	$1$
Disdyakis triacontahedron	${\frac {900+720{\sqrt {5}}}{11}}\,s^{3}$	${\frac {180{\sqrt {179-24{\sqrt {5}}}}}{11}}\,s^{2}$	${\frac {\left(\left(5+4{\sqrt {5}}\right)^{2}{\frac {11\pi }{5}}\right)^{\frac {1}{3}}}{\sqrt {179-24{\sqrt {5}}}}}\approx 0.9857$
Tricylinder	$16-8{\sqrt {2}}\,r^{3}$	$48-24{\sqrt {2}}\,r^{2}$	${\frac {\sqrt[{3}]{36\pi +18\pi {\sqrt {2}}}}{6}}\approx 0.9633$
Rhombic triacontahedron	$4{\sqrt {5+2{\sqrt {5}}}}\,s^{3}$	$12{\sqrt {5}}\,s^{2}$	${\frac {\sqrt[{6}]{455625\pi ^{2}+202500\pi ^{2}{\sqrt {5}}}}{15}}\approx 0.9609$
Icosahedron	${\frac {15+5{\sqrt {5}}}{12}}\,s^{3}$	$5{\sqrt {3}}\,s^{2}$	${\frac {\sqrt[{3}]{2100\pi {\sqrt {3}}+900\pi {\sqrt {15}}}}{30}}\approx 0.9393$
Bicylinder	${\frac {16}{3}}\,r^{3}$	$16\,r^{2}$	${\frac {\sqrt[{3}]{2\pi }}{2}}\approx 0.9226$
Ideal bicone $(h=r{\sqrt {2}})$	${\frac {2\pi }{3}}\,r^{2}h={\frac {2\pi {\sqrt {2}}}{3}}\,r^{3}$	$2\pi \,r{\sqrt {r^{2}+h^{2}}}=2\pi {\sqrt {3}}\,r^{2}$	${\frac {\sqrt[{6}]{432}}{3}}\approx 0.9165$
Dodecahedron	${\frac {15+7{\sqrt {5}}}{4}}\,s^{3}$	$3{\sqrt {25+10{\sqrt {5}}}}\,s^{2}$	${\frac {{\sqrt[{6}]{2080+928{\sqrt {5}}}}{\sqrt[{3}]{9\pi }}{\sqrt {5}}}{30}}\approx 0.9105$
Rhombic dodecahedron	${\frac {16{\sqrt {3}}}{9}}\,s^{3}$	$8{\sqrt {2}}\,s^{2}$	${\frac {\sqrt[{6}]{2592\pi ^{2}}}{6}}\approx 0.9047$
Ideal torus $(R=r)$	$2\pi ^{2}Rr^{2}=2\pi ^{2}\,r^{3}$	$4\pi ^{2}Rr=4\pi ^{2}\,r^{2}$	${\frac {\sqrt[{3}]{18\pi ^{2}}}{2\pi }}\approx 0.8947$
Ideal cylinder $(h=2r)$	$\pi \,r^{2}h=2\pi \,r^{3}$	$2\pi \,r(r+h)=6\pi \,r^{2}$	${\frac {\sqrt[{3}]{18}}{3}}\approx 0.8736$
Octahedron	${\frac {\sqrt {2}}{3}}\,s^{3}$	$2{\sqrt {3}}\,s^{2}$	${\frac {\sqrt[{3}]{3\pi {\sqrt {3}}}}{3}}\approx 0.8456$
Hemisphere	${\frac {2\pi }{3}}\,r^{3}$	$3\pi \,r^{2}$	${\frac {2{\sqrt[{3}]{2}}}{3}}\approx 0.8399$
Cube	$\,s^{3}$	$6\,s^{2}$	${\frac {\sqrt[{3}]{36\pi }}{6}}\approx 0.8060$
Ideal cone $(h=2r{\sqrt {2}})$	${\frac {\pi }{3}}\,r^{2}h={\frac {2\pi {\sqrt {2}}}{3}}\,r^{3}$	$\pi \,r(r+{\sqrt {r^{2}+h^{2}}})=4\pi \,r^{2}$	${\frac {\sqrt[{3}]{4}}{2}}\approx 0.7937$
Tetrahedron	${\frac {\sqrt {2}}{12}}\,s^{3}$	${\sqrt {3}}\,s^{2}$	${\frac {\sqrt[{3}]{12\pi {\sqrt {3}}}}{6}}\approx 0.6711$